Early observations of the increase in hydrocarbon generation with depth in sedimentary basins came from Lakaskaya and Zhabrev (1964). Vassoevich (1969) described this intensive generation zone as representing the “principal phase of oil formation”. Later, the intensive oil generation zone was known by the terms such as “oil window”, “oil kitchen”, “oil generation curve”, etc. However, Cooles
et al. (1986) pointed out that “oil generation curve” is a misnomer as the increase in oil is the net result of the three processes of oil generation, oil expulsion and oil cracking to gas, and gave the algebraic scheme based on mass balance calculations of generated and expelled petroleum.
On the other hand, Pepper (1991) and Sandvik
et al. (1992) proposed that absorption on a cross-linked polymer (kerogen) is the dominant retention mode for generated oil in source rocks.These models for the generation of oil and gas are based on two end-member scenarios: open system and closed system, which are equivalent to assuming that the residence times of the bitumen species in source rocks are either zero or infinite.
In this report, it is discussed that various features of “oil generation curve” appeared in forms such as intervals between threshold and end of intense oil generation, peak height of hydrocarbon ratio and double peak are controlled by the combination of factors such as a variation in kerogen types, geological heating rates, and the differences in absorption levels by kerogen types besides oil expulsion.
Conclusively, it is emphasized that it is important in petroleum exploration to analyze sedimentary basins, in particular, the young back-are basin of the Neogene age, Northeast Japan, from the viewpoint of whether actual petroleum basins belong to a semi-open or a semi-closed system, which will be indicated by geochemical features mentioned above.
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